使用具有默认值的参考参数是好的做法吗？

Question

我有以下代码：

#include <string>
#include <iostream>

void f(const std::string& s = "")
{
  std::cout << "\"" << s << "\"" << std::endl;
}

int main()
{
  std::string s1 = "qwe";
  f();
  f("asd");
  f(s1);
}

带有临时参数和不带参数的调用有多糟糕（如果有的话）？

据我所知，编译只是因为 const 引用延长了临时的生命，直到方法结束 http://herbsutter.com/2008/01/01/gotw-88-a-candidate-for-the-most-important-const/

尝试在s 参数旁边不带const 的情况下编译相同的示例失败。

#include <string>
#include <iostream>

void f(std::string& s = "")
{
  std::cout << "\"" << s << "\"" << std::endl;
}

int main()
{
  std::string s1 = "qwe";
  f();
  f("asd");
  f(s1);
}

编译

g++-5 -O3 -Wall --std=c++11 main.cpp  && ./a.out
main.cpp:4:27: error: invalid initialization of non-const reference of type ‘std::string& {aka std::basic_string<char>&}’ from an rvalue of type ‘std::string {aka std::basic_string<char>}’
 void f(std::string& s = "")
                           ^
In file included from /usr/include/c++/5/string:52:0,
                 from main.cpp:1:
/usr/include/c++/5/bits/basic_string.h:2893:7: note:   after user-defined conversion: std::basic_string<_CharT, _Traits, _Alloc>::basic_string(const _CharT*, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]
       basic_string(const _CharT* __s, const _Alloc& __a = _Alloc());
       ^
main.cpp: In function ‘int main()’:
main.cpp:12:5: error: invalid initialization of non-const reference of type ‘std::string& {aka std::basic_string<char>&}’ from an rvalue of type ‘std::string {aka std::basic_string<char>}’
   f();
     ^
In file included from /usr/include/c++/5/string:52:0,
                 from main.cpp:1:
/usr/include/c++/5/bits/basic_string.h:2893:7: note:   after user-defined conversion: std::basic_string<_CharT, _Traits, _Alloc>::basic_string(const _CharT*, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]
       basic_string(const _CharT* __s, const _Alloc& __a = _Alloc());
       ^
main.cpp:4:6: note: in passing argument 1 of ‘void f(std::string&)’
 void f(std::string& s = "")
      ^
main.cpp:13:10: error: invalid initialization of non-const reference of type ‘std::string& {aka std::basic_string<char>&}’ from an rvalue of type ‘std::string {aka std::basic_string<char>}’
   f("asd");
          ^
In file included from /usr/include/c++/5/string:52:0,
                 from main.cpp:1:
/usr/include/c++/5/bits/basic_string.h:2893:7: note:   after user-defined conversion: std::basic_string<_CharT, _Traits, _Alloc>::basic_string(const _CharT*, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]
       basic_string(const _CharT* __s, const _Alloc& __a = _Alloc());
       ^
main.cpp:4:6: note:   initializing argument 1 of ‘void f(std::string&)’
 void f(std::string& s = "")

Answer 1

这不是一种可怕的做法，但通常提供重载会更好：

void f(std::string const& s) { std::cout << "\\" << s << "\\\n"; }

void f() { f(""); }

它避免了一些最终让许多人感到困惑的语言功能。例如，这会打印什么？

struct base { virtual void f(int i = 42) { std::cout << i; } };

struct derived : base { void f(int i = 19) { std::cout << i; }};

int main() { base * b = new derived(); b->f(); }

当您使用默认参数时也会出现歧义错误，而使用重载时不会出现这种错误。

就特别是 const 引用而言，这并不重要。默认值在函数调用的生命周期内绑定到引用。真的一点效果都没有。当编译器可以执行某些使用引用参数无法实现的优化时，您可能有时使用值获得更好的结果，但通常不需要担心。

当然，这不适用于非常量引用，因为它们不绑定到临时对象。

Answer 2

默认值是为了让你可以不带任何参数地调用函数，所以如果你有时不给函数传参，就使用带默认值的引用参数，否则没关系。